The present invention relates to a multi-frequency antenna feed for use in providing the simultaneous reception and/or transmission of signals in at least two separate frequencies bands. The invention also relates to an apparatus and to a method of transmitting and/or receiving multiple frequency bands in a single antenna feed.
Existing antenna feeds as used in low noise blocks (LNBs) allow communication with satellites generally permit communication over a single frequency band as disclosed in applicant""s U.S. Pat. No. 5,619,173. Although the disclosed LNB works well it is nevertheless limited to a single continuous frequency band.
One of the satellite systems presently attracting interest is the Astra Return Channel System (ARCS). This involves reception from the Astra satellite system at the existing Ku-band frequencies (10.7-12.75 GHz) in both horizontal and vertical linear polarities and transmission back to satellite at Ka band (29.5-30 GHz) on a single linear polarity. Although the Astra system receives at Ku and transmits at Ka band, it is desirable to provide a system which operates equally well for reception in both bands, transmission in both bands or transmission at Ku and reception at Ka bands. Other two-way satellite communication systems around the world are proposing different frequency bands to the Astra system such as reception at one frequency band, for example 20 GHz, and transmission at a different frequency band, for example 30 GHz. For transmission or reception in widely spaced separate frequency bands using a single dish, the dish feeds for the two bands must share the same focal point. To date this has been achieved by using a coaxial waveguide structure and exciting the coaxial waveguide section with orthogonal waveguide probes (reference U.S. Pat. No. 5,463,407). This however has the disadvantage of setting up uneven fields in the coaxial waveguide which degrades the isolation between the probes and increases the probe loss. The orthogonal orientation of the probes makes it difficult to feed both polarities onto the same circuit board for subsequent processing of the received signal.
An object of the present invention is to provide a multi-frequency antenna feed which obviates or mitigates at least one of the aforementioned disadvantages.
A further object of the present invention is to provide a multi-frequency antenna feed for incorporation into a single unit which combines at least two waveguides to provide simultaneous reception and/or transmission of signals in at least two separate frequency bands.
This is achieved by creating a waveguide system of at least two waveguides sharing the same central axis; a central conventional waveguide which also acts as a center conductor for an outer coaxial waveguide and feeding the outer coaxial waveguide from an orthogonal non-circular side feed to set up a uniform field in the outer coaxial waveguide. The feeds are adjusted so that the phase center for each frequency band is at the same point in the feed for the same dish.
According to a first aspect of the present invention this is achieved by providing a single antenna feed structure having a first central waveguide operating at a first frequency band and at least one outer waveguide substantially coaxial with the central waveguide and operating at a second frequency band, said first waveguide being excited by excitation means disposed in said waveguide, and said second waveguide being excited by radiation from a non-circular waveguide disposed orthogonally to the longitudinal axis of the outer waveguide so as to set up a uniform field in said at least one outer coaxial waveguide.
Preferably, said antenna feed includes two waveguides, a first central circular waveguide and a second outer larger diameter circular waveguide coaxial with said inner central waveguide. Preferably, first frequency band is higher than the second frequency band. Alternatively, the first frequency band is lower when the central waveguide is dielectrically loaded. Alternatively, the inner waveguide has a square cross-section and the outer waveguide has a square cross-section and is coaxial with the inner waveguide. A further alternative arrangement is provided by a central circular waveguide and an outer square waveguide coaxial with the inner circular waveguide or vice versa. The inner and outer waveguide structures have cross-section which are capable of supporting two orthogonal polarisations. For example, they may be elliptical in cross-section and coaxial.
Preferably, a low pass filter is disposed between the inner and outer waveguide structures to improve signal isolation between said first and said second frequency band. Preferably, said low pass filter is provided by a plurality of spaced ridge portions upstanding from the inner coaxial waveguide.
Conveniently, there are four spaced ridge portions. Preferably also, the four ridge portions are arrange symmetrically in pairs about a plane orthogonal to the waveguide axis.
Preferably, the excitation means is a probe disposed in the central waveguide. Alternatively, the excitation means is selected from a slot radiator, a patch radiator, a dipole, a wire loop excitation probe and disposed in the central waveguide.
Preferably also, said central waveguide is fed by said probe and has a short circuit behind said probe for providing a single polarity system. Alternatively, said central waveguide has two spaced probes separated by an isolation bar, and a twist plate at the end of said waveguide for providing a dual polarity system. A dual polarity system may also be provided by using two orthogonal probes in said inner waveguide.
Preferably, the outermost waveguide is coupled to at least one rectangular waveguide to define a rectangular aperture into the coaxial guide. Conveniently, the field set up in the rectangular waveguide is achieved by using a conventional probe with a short circuit behind the probe at a nominal distance of a quarter wavelength, such that the rectangular aperture feed sets up a uniform field in the second outer coaxial waveguide. Conveniently, two rectangular feed sections are used, one for horizontal polarised signals and one for vertical polarised signals, said feeds being disposed in the same plane parallel to the waveguide axis.
Alternatively, an elliptical waveguide may be coupled to said second outer waveguide instead of a rectangular waveguide, and defining with said second waveguide an elliptical aperture in the wall of said outer waveguide. Two elliptical feed sections oriented in orthogonal directions can be used with one for horizontal signals and one for vertical signals. Alternatively, the elliptical feed sections may be in line.
As a further alternative a circular guide could be used in the side feed with a circular to rectangular or a circular to an elliptical transition to feed a corresponding rectangular or elliptical aperture in the wall of the outer coaxial waveguide.
Preferably, each of the side waveguides has a tuning post disposed therein to improve the match between the side feed waveguide and the coaxial waveguide.
Conveniently, the or each tuning post is cast into the side feed waveguide. Alternatively, the tuning posts are separate and are adjustable relative to the side feed waveguide to improve the match. Preferably, the separate tuning posts are provided by turning screws which are adjustable relative to the side feed waveguide.
The inner central waveguide preferably includes a polyrod lens for beam shaping to match up with a dish. Alternatively, a small feed horn or other type of dielectric lens may be used with the central waveguide in place of the polyrod lens.
An outer coaxial waveguide preferably opens out into a horn feed for illuminating the same dish. The horns/feeds are positioned so that the focal point for each frequency band is at the same point in the feed for the same dish. Alternatively, the horn may be replaced by a cross feed as disclosed in applicant""s co-pending published patent application No. W099/63624. Conveniently, the horn may be conical and straight sided or corrugated.
Preferably also, when the antenna feed includes waveguides which are coaxial, an isolation bar is disposed in the outer waveguide and connects between the outer surface of the inner central waveguide and the inner surface of the outer coaxial waveguide on both sides of the inner waveguide and in a plane orthogonal to the two rectangular waveguides. A single isolation bar may be used or two isolation bars next to each other or an isolation plate may be used. The isolation bar/plate serves as a short circuit for each rectangular feed and provides isolation between the two feeds.
Advantageously, a twist plate is disposed at the back of the coaxial waveguide and which extends on each side of the central waveguide. The twist plate is oriented at 45xc2x0 to the isolation bar and may take the form of any suitable twist plate such as disclosed in applicant""s published International Patent Application Nos. WO 96/28857 and WO 96/37041, that is a straight leading edge, a stepped leading edge or combination of a plate and a tapered waveguide. A tapered waveguide as disclosed in applicant""s copending application published as WO 92/22938 may be used. The tapered waveguide may be provided by providing cast stepped portions on the inner surface of the outer waveguide, said portions converging towards said inner coaxial waveguide when fitted into said outer coaxial waveguide in the antenna feed structure.
Conveniently, two stepped portions are cast into said outer waveguide.
It will also be appreciated that rectangular waveguides could feed the coaxial waveguide in orthogonal planes thus avoiding the need for a twist plate and isolation bar but making the connection of the two polarities to a circuit board more difficult.
Preferably, said inner coaxial waveguide tube is press-fitted into said outer tube. Preferably, the inner tube has its leading end coated with a conductive elastomer prior to press fitting to minimise any gaps between said inner tube and the base casting at the end of the waveguide assembly. Conveniently, said conductive elastomer is a gasket applied to the end of the inner coaxial waveguide tube.
Waveguide systems may be used which incorporate three or more waveguides, all the waveguides being coaxial with a central waveguide. The central waveguides are circular or square and the outer waveguides also circular or square. The outer waveguides are excited, as described above, by side feeds to create a uniform field in each of the outer coaxial waveguides and have waveguide components, such as probes, twist plates and isolation bars, as described above with regard to a dual coaxial waveguide arrangement, and operate in a similar manner.
According to a further aspect of the present invention, there is provided a method of providing communication from at least two separate frequency bands in a single antenna feed device, said method comprising the steps of,
providing a first central waveguide adapted to receive and/or transmit over a first frequency band, providing a second waveguide surrounding said first waveguide and coaxial with said first waveguide for receiving and/or transmitting at a second frequency band, said second frequency band being lower in frequency than said first frequency band,
exciting said first waveguide by at least a single probe disposed in said waveguide to create a uniform field within said first waveguide, and
exciting said second waveguide by feeding incident radiation into said outer waveguide in a direction orthogonal to the axis of said waveguide to create a uniform field within said outer coaxial waveguide.
The method may be used with a single or dual polarity system and may be used in any system requiring a simultaneous reception and/or transmission of signals in two separate frequency bands.